1. Field of the Invention
The present invention relates to a dielectric ceramic composition and a multilayer ceramic capacitor, and more particularly, to a dielectric ceramic composition for low-temperature sintering and hot insulation resistance (hot IR), capable of carrying out low-temperature sintering, improving a hot IR characteristic, and meeting X5R characteristics of the Electronic Industry Association (EIA) standard, and a multilayer ceramic capacitor using the same.
2. Description of the Related Art
Recently, the electric and electronic devices have been rapidly made compact, lightweight and multifunctional. Multilayer ceramic capacitors (MLCCs) used for the electric and electronic devices have also rapidly proceeded to small size and high capacity. Thus, dielectric layers used in the MLCCs have been gradually decreased in thickness but increased in the number of layers. For example, a super high-capacity capacitor is realized by stacking at least 500 dielectric layers of BaTiO3 having 2 μm or less. In order to obtain smaller scale, super high-capacity characteristics, a high-capacity dielectric layer having an ultra thin thickness of 1 μm or less must be secured.
In order to realize the MLCC using a dielectric having the ultra thin thickness, a dielectric ceramic composition that can be sintered at a temperature of 1100° C. or less is preferably used. In the case in which the dielectric ceramic composition is sintered at a temperature of 1100° C. or more, agglomeration of internal electrodes becomes serious, so that the MLCC is not only reduced in capacity but also increased in short circuit rate.
It is a hot IR characteristic that is considered when the super high-capacity MLCC is developed. When hundreds of ultra thin dielectric layers are stacked in order to obtain the super high-capacity MLCC, alternating current (AC) voltage is applied at a working temperature ranging from 100° C. to 150° C. Thereby, there is a possibility that the insulation resistance is abruptly reduced. The hot IR characteristic is used as a criterion for judging the reliability of the MLCC. Thus, the hot IR characteristic must be considered in order to realize the super high-capacity MLCC.
In order to provide the MLCC with high quality performance, the MLCC also requires temperature stability of capacitance. In addition, the MLCC requires X5R dielectric characteristics of the Electronic Industry Association (EIA) standard depending on its application. According to the EIA standard, the variation in capacitance (ΔC) should be within ±15% at a temperature ranging from −55° C. to 85° C. (when a reference temperature is 25° C.).
These characteristics can be adjusted by varying a content or percentage of each sub-component, which is added to a main component, BaTiO3, of the dielectric ceramic composition. However, one of the sub-components is added to ameliorate one of the physical properties but deteriorate another physical property. For example, the known dielectric ceramic compositions for fabricating the MLCC are used by mixing BaO and SiO2, additives (sintering agents) for facilitating sintering, at a proper ratio. In this case, the ceramic dielectric layer itself has a high dielectric constant and a good insulation resistance characteristic under conditions of high temperature and high humidity.
In this case, however, since a proper sintering temperature is usually within a range of 1150° C. to 1200° C., and since a critical temperature for low-temperature sintering is 1150° C. or so, the dielectric ceramic composition is not suitable to fabricate the MLCC having an ultra thin dielectric layer having a thickness of 1 μm or less. In contrast, in order to promote low-temperature sinterability, glass based on silicate, borosilicate or phosphate system has recently been used as the sintering agent. These dielectric ceramic compositions effectively reduce the sintering temperature, but significantly reduce the hot IR characteristic of the MLCC compared to that using BaO and SiO2 as the sintering agents.
Thus, in order to fabricate the MLCC having the super high capacity, the ultra thin thickness and numerous layers, it is necessary to develop the dielectric ceramic composition, which provides possibility of the low-temperature sintering and good hot IR characteristic as described above.